A Fully Conjugated Planar Heterocyclic [9]Circulene

Area of Science:

• Organic Chemistry
• Materials Science
• Supramolecular Chemistry

Background:

• Fully aromatic [n]circulenes, macrocyclic aromatic hydrocarbons, were previously limited to a maximum of eight aromatic rings (n=8).
• Synthesis of higher-order [n]circulenes (n>8) remained an unexplored frontier in organic chemistry.

Purpose of the Study:

• To synthesize and characterize novel, higher-order [n]circulenes beyond the previously known limit of n=8.
• To investigate the structural, electronic, and aromatic properties of these new macrocyclic systems.

Main Methods:

• A high-yielding dimerizing condensation reaction between 3,6-dihydroxycarbazole and glyoxal was employed for macrocycle construction.
• Single crystal X-ray diffraction was used to determine the precise three-dimensional structure of the synthesized [9]circulene.
• Nucleus independent chemical shift (NICS) and anisotropy of the induced current density (ACID) calculations were performed to assess aromaticity and electronic properties.

Main Results:

• The first [9]circulene (diazatrioxa[9]circulene) and a [10]circulene analogue (tetrahydro-diazatetraoxa[10]circulene) were successfully synthesized.
• X-ray analysis confirmed the [9]circulene is perfectly planar with elongated benzene rings, suggesting strain-induced planarity.
• Alternating bond lengths and paratropic ring currents in the central nonaromatic rings indicate significant [9]radialene resonance contribution and induced paratropicity.

Conclusions:

• The successful synthesis of the first [9]circulene represents a significant advancement in the field of macrocyclic aromatic compounds.
• The structural and electronic properties, including planarity and radialene character, provide new insights into the behavior of strained aromatic systems.
• The observed induced paratropicity in the nonaromatic core opens avenues for exploring novel electronic and photophysical properties in advanced materials.